1. Field of the Invention
The present invention relates to a small-sized antenna module built in a portable communication apparatus and a method for manufacturing the same.
2. Description of the Related Art
A wireless, miniature radio communication device needs an antenna for receiving and transmitting a radio wave, and generally comprises a small-sized built-in antenna having good sensitivity. As such a small-sized antenna, a planar inverted F type antenna and an S type antenna are conventionally well known.
As shown in FIG. 1, the planar inverted reserve F type antenna comprises a plate-like antenna element 12 placed on a parallel with an earth plate 11, a short pin 13 set up between the earth plate 11 and the plate-like antenna element 12, and a feeding line 14 to the plate-like antenna element 12. Input impedance to the antenna is matched by adjusting a space s between the short pin 13 and the feeding line 14. A length 1 of the plate-like antenna element 12, a width w, and a height h of the antenna are parameters of a resonance frequency. A band width becomes wider as height h is larger.
In using the planar inverted F type antenna, an ambient length of the antenna needs about a half wavelength in the basic shape. Therefore, if the antenna is miniaturized, the impedance matching between the antenna and the feeding system occasionally cannot be achieved.
As shown in FIG. 2, the S type antenna is a small-sized vertically polarized antenna, which is mounted above the upper surface of a housing 15 of the miniature radio communication apparatus. Also, the S type antenna is a top-load type antenna in which a feeding portion has a folded structure. Since a top-load portion 16 is S-like shaped, this type of antenna is called as S type antenna.
In the S type antenna, the main parameters determining an antenna characteristic are distance d between the feeding line and the short pin, a height h' of a skirt portion 17, and a gap g between the skirt portion 1 and the housing 15. The directivity of the S type antenna becomes substantially a complete round in a horizontal plane, and the gain of the S type antenna is substantially the same as that of a half wave length dipole antenna.
In the conventional planar inverted F type antenna and S type antenna, an antenna element conducting member and a ground conducting member are prepared by a plate work, and these members and an insulating member are assembled so as to have a predetermined positional relationship among them. After assembling, the dimension between the ground conducting member and the antenna element conducting member is influenced by dimensional accuracy of the plate work and the insulating member, and by the assembly accuracy of each member. Due to this, it is difficult to realize the high accuracy of the size. Therefore, the antenna characteristic varies.
Moreover, in the conventional antennas, it is required that metallic plates constituting the antenna element conducting member and the ground conducting member have thickness of 0.2 mm or more so as to maintain their shapes. This prevents the antenna from being lightened.
As mentioned above, the conventional antenna is insufficient for a built-in antenna for a miniature radio communication device in terms of the dimension accuracy, the size, and the weight, and it is difficult to realized the required performance.